1. Field of the Invention
The present invention relates to co-fired ceramic structures. More specifically, the present invention relates to displacing conductor balls into a "green" (unfired) ceramic substrate as a method of metallizing vias.
2. Description of the Prior Art and Related Information
As is known in the art, interconnections between adjacent layers in a chip package are usually performed by vias formed into the substrate. Typically, the vias are created by either laser vaporization or by punching. The subsequent metallization step involves screen printing a thick film metal paste on to the substrate, which along the way then fills in the vias.
A drawback with the screen printing process is that it is a relatively slow process and the result is dependent on many sensitive factors. A certain amount of skilled labor is involved, which in turn has a toll on process speed and quality. Correspondingly, the conventional screen printing process is a relatively expensive way to metallize vias.
Conventional silk screen processes often produce poor quality conductive vias because thick film conductive pastes are used. These pastes have variable shrinkage and often cause incomplete or overfilled vias. In the incomplete filling case, where the defect is called a "dimple," an insufficient amount of paste fills in the via and upon heating, the liquified paste sags into the via giving the appearance of a dimple. If too much conductive paste is used to fill the via, wherein the defect is called a "post", aggregated paste builds into a mound that protrudes above the via. In either case, the finished substrate surface is very rough and provides a bad metallization surface for the next layer. Also, because the rough surface is not flat, any wire bonding or attachment of interconnects or surface mount devices becomes difficult. Indeed, as is known in the art, surface smoothness has an effect on good bonds.
Also with prior art silk screen processes, as mentioned above, because the dielectric substrate and the conductor paste each expands and contracts at different rates during processing due to different coefficients of thermal expansion, the variations in expansion/shrink rates cause unevenness in the finished substrate surface. A major part of the variations stems from the organic binder blended in the paste, which burns off during the firing process thereby causing large dimensional shrinkage in the metallized vias. The organic binder is needed to carry the paste through the mesh openings, so it is an integral part of the screen printing process. On the other hand, the present invention utilizes metal spheres and does not require use of the organic binder.
Therefore, a need presently exists for efficiently metallizing vias in a substrate with high quality yields while maintaining low manufacturing costs.